A color liquid crystal display has conventionally included normally three pixels (picture elements) of R, G, and B in one (1) dot (pixel). Recent years have witnessed, however, a proposal of a color liquid crystal display that additionally includes a white (W) picture element for improved luminance, the color liquid crystal display thus including a total of four picture elements of R, G, B, and W in one (1) dot. This RGBW color liquid crystal display is advantageous in terms of luminance improvement, but tends to have chroma decrease. An RGBW color liquid crystal display thus requires careful selection of a method for determining, for example, the structure and output gray-scale level of a W picture element.
Patent Literature 1, for example, discloses information on the structure of a W picture element. Specifically, it proposes reducing the size of a W picture element so that it is smaller than the size of any of R, G, and B picture elements. This intends to prevent chroma decrease resulting from a display signal for a W picture element.
Patent Literature 1 further discusses converting input RGB data into RGBW data and then converting the RGBW data into optimized data R′G′B′W′. After so converting RGB data into RGBW data (by a method of which no detailed description is provided), the technique of Patent Literature 1 further converts the resulting RGBW data as above in order to both improve luminance and maintain chroma.
A W picture element being smaller in size than R, G, and B picture elements, however, leads to the luminance improvement effect being lessened accordingly. Patent Literature 1 discloses in an Example a calculation result of a 50% luminance increase. This calculation result is, however, for the case in which R, G, B, and W picture elements all have an equal size. The technique of Patent Literature 1 merely produces a luminance increase of approximately 42% in the case where, for instance, R=G=B=1.05 and W=0.85 with the index of 1 being for the original size of a picture element of each color. This indicates that the technique of Patent Literature 1 can maintain chroma, but cannot improve luminance sufficiently.
In addition, converting R, G, and B input signals into R, G, B, and W signals and then further converting the R, G, B, and W signals into signals of R′, G′, B′, and W′ forces a driving circuit to internally carry out arithmetic processing in an even larger amount, resulting in (i) the circuit being overly large and (ii) a cost increase.
Patent Literature 2 discloses a most convenient technique (method) for calculating R, G, B, and W signals from R, G, and B signals. This technique is characterized to involve (i) means (min detecting section 1) for extracting a white-color signal from a plurality of color component signals inputted and (ii) means for outputting, on the basis of the extracted white-color signal and the plurality of color component signals, display signals for four respective colors including at least white. This technique is also characterized to involve means for nonlinearly processing the extracted white-color signal and outputting display signals on the basis of the plurality of color component signals and the nonlinearly converted white-color signal.
The above method extracts a minimum value among the respective values of R, G, and B signals as a white signal W, and nonlinearly converts W into W′ as necessary. The above method thus carries out the following calculation:OUT(R,G,B)=IN(R,G,B)−OUT(W), andOUT(W)=W or W′
The above method inevitably causes at least one of R, G, and B outputs to be 0 if W is not nonlinearly processed. During a white or gray display in particular, Rout=Gout=Bout=0 and W=min(Rin, Gin, Bin), which leads to no luminance improvement and to significant chroma decrease. Nonlinearly processing W into W′ may alleviate the problem of chroma decrease, but still fails to improve luminance because carrying out nonlinear processing does not mean carrying out luminance expansion with respect to input signals.
Patent Literature 3, for example, discloses a more complex technique for calculating R, G, B, and W signals from R, G, and B signals. Patent Literature 3 proposes a method for determining a white-color signal component, the method including (i) a step of determining the amount of luminance increase separately for each of the reference color components included in predetermined color signals, (ii) a step of extracting data on the amount of luminance increase for a white-color signal component from the respective amounts of luminance increase for the reference color components, and (iii) a step of setting the amount of luminance increase for a white-color signal component as a white-color signal component for the predetermined color signals.
The description below deals in detail with the technique of Patent Literature 3 with reference to FIG. 9. FIG. 9 illustrates only a case of a system involving two signals of R and G for simplification. In the case where, for instance, R is a minimum luminance signal Lmin and G is a maximum luminance signal Lmax, the technique of Patent Literature 3 carries out luminance expansion for (i) color A to produce a maximum luminance A′ on a O-A extension and also for (ii) color C, which has an R-G luminance ratio equal to that of color A, at S=OA′/OA. This technique employs the expansion rate of S=OB′/OB for color D similarly. Thus, the luminance expansion rate S and t (=Lmin/Lmax) have a relation illustrated in FIG. 10.
The conventional technique disclosed in Patent Literature 3, however, problematically decreases the quality of a display image. Specifically, the above conventional technique poses the following problems: First, the conventional technique carries out no luminance expansion with respect to primary colors. Thus, in the case where a primary color and white are adjacent to each other, the primary color is seen as having chroma decrease, thereby decreasing the quality of a display image. Second, the conventional technique involves a luminance expansion rate S that is, as illustrated in FIG. 10, saturated at t=0.5 and that has a curve which is bent abruptly. This leads to the presence of a pattern at which a video image is seen unnaturally, thereby decreasing the quality of a display image.
As described above, the respective techniques of Patent Literatures 1 to 3 are each problematic in that it cannot improve luminance while maintaining the hues of a video image and decreases the quality of a display image when determining color signals of R, G, B, and W from color signals of R, G, and B.
Patent Literature 4 discloses a technique that solves the above problem. This technique of Patent Literature 4 determines color signals of R, G, B, and W from color signals of R, G, and B as follows: The technique carries out luminance compression with respect to color signals of R, G, and B for the current frame at a predetermined luminance compression ratio selected in correspondence with the respective luminances of color signals of R, G, B, and W for the last frame to determine post-compression color signals of R, G, and B. The technique next determines a luminance expansion rate on the basis of the ratio between the value of a minimum luminance and that of a maximum luminance of the post-compression color signals, and carries out luminance expansion with respect to the post-compression color signals at the luminance expansion rate to determine luminance-expanded color signals of R, G, and B. The technique then multiplies the minimum luminance value by a coefficient k to determine a corrected minimum luminance value, and subtracts the corrected minimum luminance value from each of the luminance-expanded color signals to determine luminance-expanded converted color signals of R, G, and B. The technique also sets a white-color (W) signal to have the minimum luminance value. The technique thus uses the luminance-expanded converted color signals of R, G, and B and the white-color signal as color signals of R, G, B, and W.
Patent Literature 4 solves the above problem by determining color signals of R, G, B, and W from color signals of R, G, and B as described above.
The technique of Patent Literature 4 controls the predetermined luminance compression ratio through feedback processing to an optimum value in order to prevent a predetermined number or more of luminance-saturated dots from being present in an image represented by color signals of R, G, B, and W (hereinafter referred to as “converted image”).
In other words, the technique of Patent Literature 4, (i) in the case where there are present a predetermined number or more of luminance-saturated dots in a converted image for the current frame, decreases the value of the predetermined luminance compression ratio for use in determining a converted image for the subsequent frame, and (ii) otherwise increases the value of the predetermined luminance compression ratio for use in determining a converted image for the subsequent frame.